High temperature oxidation protection for composites

ABSTRACT

The present disclosure provides a method for coating a composite structure, comprising applying a single pretreating composition on a surface of the composite structure, the single pretreating composition comprising a first acid aluminum phosphate comprising a molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3, and heating the composite structure to a first temperature sufficient to form an aluminum phosphate polymer layer on the composite structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, claims priority to and the benefitof, U.S. Ser. No. 15/194,034 filed Jun. 27, 2016 and entitled “HIGHTEMPERATURE OXIDATION PROTECTION FOR COMPOSITES,” which is herebyincorporated by reference.

FIELD

The present disclosure relates generally to carbon-carbon compositesand, more specifically, to oxidation protection systems forcarbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typicallydesigned to minimize loss of carbon material due to oxidation atoperating conditions, which include temperatures as high as 900° C.(1652° F.). Phosphate-based oxidation protection systems may reduceinfiltration of oxygen and oxidation catalysts into the compositestructure. However, despite the use of such oxidation protectionsystems, significant oxidation of the carbon-carbon composites may stilloccur during operation of components such as, for example, aircraftbraking systems.

SUMMARY

A method for coating a composite structure is provided, comprisingapplying a single pretreating composition on a surface of the compositestructure, the single pretreating composition comprising a first acidaluminum phosphate comprising a first molar ratio of aluminum tophosphate between 1 to 2 and 1 to 3, and heating the composite structureto a first temperature sufficient to form an aluminum phosphate polymerlayer on the composite structure. In various embodiments, the method mayfurther comprise applying a second slurry to the aluminum phosphatepolymer layer, and heating the composite structure to a secondtemperature sufficient to form a sealing layer on the aluminum phosphatepolymer layer.

In various embodiments, the second slurry may comprise a secondpre-slurry composition and a second carrier fluid, wherein the secondpre-slurry composition comprises a second phosphate glass composition.In various embodiments, the method may further comprise applying a firstslurry to the aluminum phosphate polymer layer and heating the compositestructure to a third temperature sufficient to form a base layer priorto the applying the second slurry to the aluminum phosphate polymerlayer, wherein the first slurry comprises a first pre-slurry compositionand a first carrier fluid, wherein the first pre-slurry compositioncomprises a first phosphate glass composition. In various embodiments,the first pre-slurry composition may comprise a second acid aluminumphosphate wherein a second molar ratio of aluminum to phosphate isbetween 1 to 2 and 1 to 3. In various embodiments, the first molar ratioof the aluminum to phosphate in the first acid aluminum phosphate may bebetween 1 to 2 and 1 to 2.7.

In various embodiments, the method may further comprise applying abarrier coating to the composite structure prior to the applying thesingle pretreating composition to the composite structure. The barriercoating may comprise at least one of a carbide, a nitride, a boronnitride, a silicon carbide, a titanium carbide, a boron carbide, asilicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or asilicon nitride.

In various embodiments, at least one of the first phosphate glasscomposition or the second phosphate glass composition may be representedby the formula a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, andmixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium,manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead,zirconium, lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium,calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

In various embodiments, the first slurry may comprise a refractorycompound such as a nitride, a boron nitride, a silicon carbide, atitanium carbide, a boron carbide, a silicon oxycarbide, siliconnitride, molybdenum disulfide, or tungsten disulfide. In variousembodiments, at least one of the first slurry or the second slurry maycomprise at least one of a surfactant, a flow modifier, a polymer,ammonium hydroxide, ammonium dihydrogen phosphate, acid aluminumphosphate, nanoplatelets, or graphene nanoplatelets.

In various embodiments, a single pretreating composition may comprise anacid aluminum phosphate, wherein the acid aluminum phosphate maycomprise a molar ratio of aluminum to phosphate is between 1 to 2 and 1to 3. In various embodiments, the single pretreating composition may besubstantially free of aluminum oxide. In various embodiments, the molarratio of aluminum to phosphate in the acid aluminum phosphate may bebetween 1 to 2 and 1 to 2.7. In various embodiments, the molar ratio ofaluminum to phosphate in the acid aluminum phosphate may be 1 to 2.5.

In various embodiments, an article may comprise a carbon-carboncomposite structure, an oxidation protection system including analuminum phosphate polymer layer disposed on an outer surface of thecarbon-carbon composite structure, the aluminum phosphate polymer layerhaving a first molar ratio of aluminum to phosphate between 1 to 2 and 1to 3. In various embodiments, the article may further comprise a sealinglayer disposed on the aluminum phosphate polymer layer. The sealinglayer may comprise a second phosphate glass composition. In variousembodiments, the oxidation protection system may further comprise a baselayer disposed between the aluminum phosphate polymer layer and thesealing layer, wherein the base layer may comprise a first phosphateglass composition and an acid aluminum phosphate. In variousembodiments, the base layer may comprise a second molar ratio ofaluminum to phosphate between 1 to 2 and 1 to 3.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present disclosure, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIGS. 2A, 2B, and 2C illustrate methods for coating a compositestructure, in accordance with various embodiments;

FIG. 3 illustrates experimental data obtained from testing various glasscompositions, in accordance with various embodiments;

FIG. 4 illustrates further experimental data obtained from testingvarious glass compositions, in accordance with various embodiments at760° C. (1400° F.);

FIG. 5 illustrates further experimental data obtained from testingvarious oxidation protection slurries, in accordance with variousembodiments at 760° C. (1400° F.); and

FIG. 6 illustrates further experimental data obtained from testingvarious oxidation protection systems, in accordance with variousembodiments at 760° C. (1400° F.).

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to theaccompanying drawings, which show embodiments by way of illustration.While these embodiments are described in sufficient detail to enablethose skilled in the art to practice the disclosure, it should beunderstood that other embodiments may be realized and that logical andmechanical changes may be made without departing from the spirit andscope of the disclosure. Thus, the detailed description herein ispresented for purposes of illustration only and not for limitation. Forexample, any reference to singular includes plural embodiments, and anyreference to more than one component or step may include a singularembodiment or step. Also, any reference to attached, fixed, connected orthe like may include permanent, removable, temporary, partial, fulland/or any other possible attachment option.

With initial reference to FIGS. 1A and 1B, aircraft wheel brakingassembly 10 such as may be found on an aircraft, in accordance withvarious embodiments is illustrated. Aircraft wheel braking assembly may,for example, comprise a bogie axle 12, a wheel 14 including a hub 16 anda wheel well 18, a web 20, a torque take-out assembly 22, one or moretorque bars 24, a wheel rotational axis 26, a wheel well recess 28, anactuator 30, multiple brake rotors 32, multiple brake stators 34, apressure plate 36, an end plate 38, a heat shield 40, multiple heatshield sections 42, multiple heat shield carriers 44, an air gap 46,multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52,multiple heat shield fasteners 53, multiple rotor lugs 54, and multiplestator slots 56. FIG. 1B illustrates a portion of aircraft wheel brakingassembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel brakingassembly 10 may be subjected to the application of compositions andmethods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed inwheel well recess 28 of wheel well 18. Rotors 32 are secured to torquebars 24 for rotation with wheel 14, while stators 34 are engaged withtorque take-out assembly 22. At least one actuator 30 is operable tocompress interleaved rotors 32 and stators 34 for stopping the aircraft.In this example, actuator 30 is shown as a hydraulically actuatedpiston, but many types of actuators are suitable, such as anelectromechanical actuator. Pressure plate 36 and end plate 38 aredisposed at opposite ends of the interleaved rotors 32 and stators 34.Rotors 32 and stators 34 can comprise any material suitable for frictiondisks, including ceramics or carbon materials, such as a carbon/carboncomposite.

Through compression of interleaved rotors 32 and stators 34 betweenpressure plates 36 and end plate 38, the resulting frictional contactslows rotation of wheel 14. Torque take-out assembly 22 is secured to astationary portion of the landing gear truck such as a bogie beam orother landing gear strut, such that torque take-out assembly 22 andstators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as compositestructures, composite substrates, and carbon-carbon compositestructures, interchangeably) in the friction disks may operate as a heatsink to absorb large amounts of kinetic energy converted to heat duringslowing of the aircraft. Heat shield 40 may reflect thermal energy awayfrom wheel well 18 and back toward rotors 32 and stators 34. Withreference to FIG. 1A, a portion of wheel well 18 and torque bar 24 isremoved to better illustrate heat shield 40 and heat shield segments 42.With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and isconcentric with wheel well 18. Individual heat shield sections 42 may besecured in place between wheel well 18 and rotors 32 by respective heatshield carriers 44 fixed to wheel well 18. Air gap 46 is definedannularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14using bolts or other fasteners. Torque bar bolts 48 can extend through ahole formed in a flange or other mounting surface on wheel 14. Eachtorque bar 24 can optionally include at least one torque bar pin 50 atan end opposite torque bar bolts 48, such that torque bar pin 50 can bereceived through wheel web hole 52 in web 20. Heat shield sections 42and respective heat shield carriers 44 can then be fastened to wheelwell 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraftwheel braking assembly 10, carbon-carbon composites may be prone tomaterial loss from oxidation of the carbon. For example, variouscarbon-carbon composite components of aircraft wheel braking assembly 10may experience both catalytic oxidation and inherent thermal oxidationcaused by heating the composite during operation. In variousembodiments, composite rotors 32 and stators 34 may be heated tosufficiently high temperatures that may oxidize the carbon surfacesexposed to air. At elevated temperatures, infiltration of air andcontaminants may cause internal oxidation and weakening, especially inand around brake rotor lugs 54 or stator slots 56 securing the frictiondisks to the respective torque bar 24 and torque take-out assembly 22.Because carbon-carbon composite components of aircraft wheel brakingassembly 10 may retain heat for a substantial time period after slowingthe aircraft, oxygen from the ambient atmosphere may react with thecarbon matrix and/or carbon fibers to accelerate material loss. Further,damage to brake components may be caused by the oxidation enlargement ofcracks around fibers or enlargement of cracks in a reaction-formedporous barrier coating (e.g., a silicon-based barrier coating) appliedto the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carboncomposite brake components include potassium (K) and sodium (Na). Thesealkali contaminants may come into contact with aircraft brakes as partof cleaning or de-icing materials. Other sources include salt depositsleft from seawater or sea spray. These and other contaminants (e.g. Ca,Fe, etc.) can penetrate and leave deposits in pores of carbon-carboncomposite aircraft brakes, including the substrate and anyreaction-formed porous barrier coating. When such contamination occurs,the rate of carbon loss by oxidation can be increased by one to twoorders of magnitude.

In various embodiments, components of aircraft wheel braking assembly 10may reach operating temperatures in the range from about 100° C. (212°F.) up to about 900° C. (1652° F.). However, it will be recognized thatthe oxidation protection systems and methods of the present disclosuremay be readily adapted to many parts in this and other brakingassemblies, as well as to other carbon-carbon composite structuressusceptible to oxidation losses from infiltration of atmospheric oxygenand/or catalytic contaminants. An oxidation protection system maycomprise various layers being applied to a composite structure, such aslayers formed by a pretreating composition, a first slurry to form abase layer, a second slurry to form a sealing layer, and/or variousother layers.

In various embodiments, a method for limiting an oxidation reaction in acomposite structure may comprise forming a first slurry by combining afirst pre-slurry composition comprising a first phosphate glasscomposition in the form of a glass frit, powder, or other suitablepulverized form, with a first carrier fluid (such as, for example,water), applying the first slurry to a composite structure, and heatingthe composite structure to a temperature sufficient to dry the carrierfluid and form an oxidation protection coating on the compositestructure, which in various embodiments may be referred to a base layer.The first pre-slurry composition of the first slurry may compriseadditives, such as, for example, ammonium hydroxide, ammonium dihydrogenphosphate, nanoplatelets (such as graphene-based nanoplatelets), amongothers, to improve hydrolytic stability and/or to increase the compositestructure's resistance to oxidation, thereby tending to reduce mass lossof composite structure. In various embodiments, a slurry comprising acidaluminum phosphates having an aluminum (Al) to phosphoric acid (H₃PO₄)molar ratio of 1 to 3 or less, such as an Al:H₃PO₄ molar ratio ofbetween 1 to 2 and 1 to 3, tends to provide increased hydrolyticstability without substantially increasing composite structure massloss. In various embodiments, a slurry comprising acid aluminumphosphates having an Al:H₃PO₄ molar ratio between 1:2 to 1:3 produces anincrease in hydrolytic protection and an unexpected reduction incomposite structure mass loss.

With initial reference to FIG. 2A, a method 200 for coating a compositestructure in accordance with various embodiments is illustrated. Method200 may, for example, comprise applying an oxidation protection systemto non-wearing surfaces of carbon-carbon composite brake components. Invarious embodiments, method 200 may be used on the back face of pressureplate 36 and/or end plate 38, an inner diameter (ID) surface of stators34 including slots 56, as well as outer diameter (OD) surfaces of rotors32 including lugs 54. The oxidation inhibiting composition of method 200may be applied to preselected regions of a carbon-carbon compositestructure that may be otherwise susceptible to oxidation. For example,aircraft brake disks may have the oxidation inhibiting compositionapplied on or proximate stator slots 56 and/or rotor lugs 54.

In various embodiments, method 200 may comprise forming a first slurry(step 210) by combining a first pre-slurry composition, comprising afirst phosphate glass composition in the form of a glass frit, powder,or other suitable pulverized and/or ground form, with a first carrierfluid (such as, for example, water). In various embodiments, the firstslurry may comprise an acid aluminum phosphate wherein the molar ratioof Al:H₃PO₄ may be between 1:2 to 1:3, between 1:2.2 to 1:3, between1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. The firstpre-slurry composition of the first slurry may further comprise a boronnitride additive. For example, a boron nitride (such as hexagonal boronnitride) may be added to the first phosphate glass composition such thatthe resulting first pre-slurry composition comprises between about 10weight percent and about 30 weight percent of boron nitride, wherein theterm “about” in this context only means plus or minus 2 weight percent.Further, the pre-slurry composition may comprise between about 15 weightpercent and 25 weight percent of boron nitride, wherein the term “about”in this context only means plus or minus 2 weight percent. Boron nitridemay be prepared for addition to the first phosphate glass compositionby, for example, ultrasonically exfoliating boron nitride indimethylformamide (DMF), a solution of DMF and water, or 2-propanolsolution. In various embodiments, the boron nitride additive maycomprise a boron nitride that has been prepared for addition to thefirst phosphate glass composition by crushing or milling (e.g., ballmilling) the boron nitride. The resulting boron nitride may be combinedwith the first phosphate glass composition glass frit.

The first phosphate glass composition may comprise one or more alkalimetal glass modifiers, one or more glass network modifiers and/or one ormore additional glass formers. In various embodiments, boron oxide or aprecursor may optionally be combined with the P₂O₅ mixture to form aborophosphate glass, which has improved self-healing properties at theoperating temperatures typically seen in aircraft braking assemblies. Invarious embodiments, the phosphate glass and/or borophosphate glass maybe characterized by the absence of an oxide of silicon. Further, theratio of P₂O₅ to metal oxide in the fused glass may be in the range fromabout 0.25 to about 5 by weight.

Potential alkali metal glass modifiers may be selected from oxides oflithium, sodium, potassium, rubidium, cesium, and mixtures thereof. Invarious embodiments, the glass modifier may be an oxide of lithium,sodium, potassium, or mixtures thereof. These or other glass modifiersmay function as fluxing agents. Additional glass formers can includeoxides of boron, silicon, sulfur, germanium, arsenic, antimony, andmixtures thereof.

Suitable glass network modifiers include oxides of vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof.

The first phosphate glass composition may be prepared by combining theabove ingredients and heating them to a fusion temperature. In variousembodiments, depending on the particular combination of elements, thefusion temperature may be in the range from about 700° C. (1292° F.) toabout 1500° C. (2732° F.). The resultant melt may then be cooled andpulverized and/or ground to form a glass frit or powder. In variousembodiments, the first phosphate glass composition may be annealed to arigid, friable state prior to being pulverized. Glass transitiontemperature (T_(g)), glass softening temperature (T_(s)) and glassmelting temperature (T_(m)) may be increased by increasing refinementtime and/or temperature. Before fusion, the first phosphate glasscomposition comprises from about 20 mol % to about 80 mol % of P₂O₅. Invarious embodiments, the first phosphate glass composition comprisesfrom about 30 mol % to about 70 mol % P₂O₅, or precursor thereof. Invarious embodiments, the first phosphate glass composition comprisesfrom about 40 to about 60 mol % of P₂O₅.

The first phosphate glass composition may comprise from about 5 mol % toabout 50 mol % of the alkali metal oxide. In various embodiments, thefirst phosphate glass composition comprises from about 10 mol % to about40 mol % of the alkali metal oxide. Further, the first phosphate glasscomposition comprises from about 15 to about 30 mol % of the alkalimetal oxide or one or more precursors thereof. In various embodiments,the first phosphate glass composition may comprise from about 0.5 mol %to about 50 mol % of one or more of the above-indicated glass formers.The first phosphate glass composition may comprise about 5 to about 20mol % of one or more of the above-indicated glass formers. As usedherein, mol % is defined as the number of moles of a constituent per thetotal moles of the solution.

In various embodiments, the first phosphate glass composition cancomprise from about 0.5 mol % to about 40 mol % of one or more of theabove-indicated glass network modifiers. The first phosphate glasscomposition may comprise from about 2.0 mol % to about 25 mol % of oneor more of the above-indicated glass network modifiers.

In various embodiments, the first phosphate glass composition, excludingthe low CTE material, may represented by the formula:

a(A′₂O)_(x)(P₂O₅)_(y1) b(G_(f)O)_(y2) c(A″O)_(z)  [1]

In Formula 1, A′ is selected from: lithium, sodium, potassium, rubidium,cesium, and mixtures thereof; G_(f) is selected from: boron, silicon,sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ isselected from: vanadium, aluminum, tin, titanium, chromium, manganese,iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,actinium, thorium, uranium, yttrium, gallium, magnesium, calcium,strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is anumber in the range from 1 to about 5; b is a number in the range from 0to about 10; c is a number in the range from 0 to about 30; x is anumber in the range from about 0.050 to about 0.500; y₁ is a number inthe range from about 0.100 to about 0.950; y₂ is a number in the rangefrom 0 to about 0.20; and z is a number in the range from about 0.01 toabout 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂). The first phosphate glasscomposition may be formulated to balance the reactivity, durability andflow of the resulting glass barrier layer for optimal performance.

In various embodiments, first phosphate glass composition in glass fritform may be combined with additional components to form the firstpre-slurry composition. For example, crushed first phosphate glasscomposition in glass frit form may be combined with ammonium hydroxide,ammonium dihydrogen phosphate, nanoplatelets (such as graphene-basednanoplatelets), among other materials and/or substances. For example,graphene nanoplatelets could be added to the first phosphate glasscomposition in glass frit form. In various embodiments, the additionalcomponents may be combined and preprocessed before combining them withfirst phosphate glass composition in glass frit form. Other suitableadditional components include, for example, surfactants such as, forexample, an ethoxylated low-foam wetting agent and flow modifiers, suchas, for example, polyvinyl alcohol, polyacrylate, or similar polymers.In various embodiments, other suitable additional components may includeadditives to enhance impact resistance and/or to toughen the barriercoating, such as, for example, at least one of whiskers, nanofibers ornanotubes consisting of nitrides, carbides, carbon, graphite, quartz,silicates, aluminosilicates, phosphates, and the like. In variousembodiments, additives to enhance impact resistance and/or to toughenthe barrier coating may include silicon carbide whiskers, carbonnanofibers, boron nitride nanotubes and similar materials known to thoseskilled in the art.

The components in the first pre-slurry composition may have coefficientsof thermal expansion (“CTE”) that are significantly higher than thecoefficient of thermal expansion of carbon composite material in acomposite structure, to which the first slurry may be applied. The CTEof carbon composite materials may range from below zero to about3.6×10⁻⁶° C.⁻¹. The CTE of phosphate glass, on the other hand, is higherthan that of carbon composite materials, ranging from about 12×10⁻⁶°C.⁻¹ to about 14×10⁻⁶° C.⁻¹, wherein the term “about” as used in thiscontext only refers to plus or minus 2×10⁻⁶° C.⁻¹. Therefore, duringoperation, the thermal expansion of phosphate glass in the firstpre-slurry composition of the first slurry may cause cracks in thecarbon material in the composite structure, because the carbon materialexpands at a slower rate than the phosphate glass, and other materialsin the first slurry, when exposed to heat.

In various embodiments, materials with low CTEs may be added to thefirst pre-slurry composition. Materials with low CTEs may be materialshaving CTEs of 10×10⁻⁶° C.⁻¹ or less, or further, materials having CTEsof 3×10⁻⁶° C.⁻¹ or less. For example, a low CTE material added to thefirst pre-slurry composition may be beta-spodumene (Li₂O*Al₂0₃*2SiO₂)with a CTE of 0.9×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.),beta-eucryptite (Li₂O*Al₂O₃*2SiO₂) with a CTE of −6.2×10⁻⁶° C.⁻¹ at 25°C. (77° F.) to 1000° C. (1832° F.), cordierite (Mg₂Al₄Si₅O₁₈) with a CTEof 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C. (1472° F.), faujasite((Na₂,Ca,Mg)_(3.5)[Al₇Si₁₇O₄₈].32(H₂O)) with a CTE of −3.24×10⁻⁶° C.⁻¹,NZP materials such as SrZr₄P₆O₂₄ and CaZr₄P₆O₂₄ with CTEs of 0.6×10⁻⁶°C. at 25° C. (77° F.) to 1000° C. (1832° F.) and NaZr₂P₃O₁₂ with a CTEof −0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), Al₂Mo₃O₁₂with a CTE of 2.4×10⁻⁶° C.⁻¹, Al₂W₃O₁₂ with a CTE of 2.2×10⁻⁶° C.⁻¹,MoZr₂P₂O₁₂, WZr₂P₂O₁₂, niobium pentoxide (Nb₂O₅) with a CTE of 1.0×10⁻⁶°C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), aluminum titanate(Al₂TiO₅) with a CTE of 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C.(1472° F.), Zr₂P₂O₉ with a CTE of 0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to600° C. (1112° F.), Beryl (Be₃Al₂Si₆O₁₈) with a CTE of 2.0×10⁻⁶° C.⁻¹ at25° C. (77° F.) to 1000° C. (1832° F.), silicon dioxide glass (SiO₂)with a CTE of 0.5×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.),SiO₂—TiO₂ glass with a CTE of −0.03×10⁻⁶° C.⁻¹ to 0.05×10⁻⁶° C.⁻¹ at 25°C. (77° F.) to 800° C. (1472° F.), Cu₂O—Al₂O₃—SiO₂ glasses with a CTE of0.5×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 500° C. (932° F.), ZERODUR (amaterial comprising, by weight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5% P₂O₅, 3.4%Li₂O, 2.3% TiO₂, 1.8% ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃, 0.4% K₂O,and 0.2% Na₂O) with a CTE of 0.12×10⁻⁶° C.⁻¹ at 20° C. (68° F.) to 600°C. (1112° F.), and/or lead magnesium niobate (Pb₃MgNb₂O₉) with a CTE of1.0×10⁻⁶° C.⁻¹ at −100° C. (−148° F.) to 100° C. (212° F.). In variousembodiments, the first slurry may comprise an amount between 0.5% byweight and 50% by weight of the low CTE material. In variousembodiments, the first slurry may comprise an amount between 0.5% byweight and 40% by weight of the low CTE material. In variousembodiments, the first slurry may comprise an amount between 5% byweight and 30% by weight of the low CTE material. In variousembodiments, the first pre-slurry composition may comprise between 0.5%by weight and 95% by weight of the low CTE material, wherein the firstpre-slurry composition comprises all the components of the first slurryexcept the first carrier fluid. In various embodiments, the firstpre-slurry composition may comprise between 1% by weight and 90% byweight of the low CTE material. In various embodiments, the firstpre-slurry composition may comprise between 5% by weight and 80% byweight of the low CTE material. In various embodiments, the firstpre-slurry composition may comprise between 10% by weight and 30% byweight of the low CTE material.

By adding a low CTE material, such as those listed herein, to the firstpre-slurry composition, the difference between the CTE of the carbonmaterial in the composite structure and the average CTE of the firstslurry, which will form the base layer after heating (step 230,described below), may be decreased. Therefore, by tailoring the thermalexpansion properties of the base layer through the addition of one ormore low CTE materials, in response to being exposed to heat, the baselayer and the carbon material in the composite structure may expand atmore similar rates than a base layer without a low CTE material. Moresimilar CTEs between the carbon material in the composite structure andthe base layer formed by the first slurry may help maintain thestructural integrity of the composite structure by alleviating crackingin the carbon material and/or cracking in the base layer in response tobeing heated.

In various embodiments, method 200 further comprises applying the firstslurry to a composite structure (step 220). Applying the first slurrymay comprise, for example, spraying or brushing the first slurry of thefirst phosphate glass composition on to an outer surface of thecomposite structure. Any suitable manner of applying the base layer tothe composite structure is within the scope of the present disclosure.As referenced herein, the composite structure may refer to acarbon-carbon composite structure.

In various embodiments, method 200 further comprises a step 230 ofheating the composite structure to a temperature sufficient to form abase layer of phosphate glass, which may be referred to as a base layertemperature. The composite structure may be heated (e.g., dried orbaked) at a temperature in the range from about 200° C. (292° F.) toabout 1000° C. (1832° F.). In various embodiments, the compositestructure is heated to a temperature in a range from about 600° C.(1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (292°F.) to about 900° C. (1652° F.), or further, between about 400° C. (752°F.) to about 850° C. (1562° F.). Step 230 may, for example, compriseheating the composite structure for a period between about 0.5 hour andabout 8 hours, wherein the term “about” in this context only means plusor minus 0.25 hours. The base layer may also be referred to as acoating.

In various embodiments, the composite structure may be heated to a lowertemperature first (for example, about 30° C. (86° F.) to about 400° C.(752° F.)) to bake or dry the base layer at a controlled depth. Asecond, higher temperature (for example, about 300° C. (572° F.) toabout 1000° C. (1832° F.)) may then be used to form a deposit from thebase layer within the pores of the composite structure. The duration ofeach heating step can be determined as a fraction of the overall heatingtime and can range from about 10% to about 50%, wherein the term “about”in this context only means plus or minus 5%. In various embodiments, theduration of the lower temperature heating step(s) can range from about20% to about 40% of the overall heating time, wherein the term “about”in this context only means plus or minus 5%. The lower temperaturestep(s) may occupy a larger fraction of the overall heating time, forexample, to provide relatively slow heating up to and through the firstlower temperature. The exact heating profile will depend on acombination of the first lower temperature and desired depth of thedrying portion.

Step 230 may be performed in an inert environment, such as under ablanket of inert gas or less reactive gas (e.g., nitrogen (N₂), argon,other noble gases and the like). For example, a composite structure maybe pretreated or warmed prior to application of the base layer to aid inthe penetration of the base layer. Step 230 may be for a period of about2 hours at a temperature of about 600° C. (1112° F.) to about 800° C.(1472° F.), wherein the term “about” in this context only means plus orminus 10° C. The composite structure and base layer may then be dried orbaked in a non-oxidizing, inert or less reactive atmosphere, e.g., noblegasses and/or nitrogen (N₂), to optimize the retention of the firstpre-slurry composition of the base layer in the pores of the compositestructure. This retention may, for example, be improved by heating thecomposite structure to about 200° C. (392° F.) and maintaining thetemperature for about 1 hour before heating the carbon-carbon compositeto a temperature in the range described above. The temperature rise maybe controlled at a rate that removes water without boiling, and providestemperature uniformity throughout the composite structure.

In various embodiments and with reference now to FIG. 2B, method 300,which comprises steps also found in method 200, may further compriseapplying at least one of a pretreating composition or a barrier coating(step 215) prior to applying the first slurry. Step 215 may, forexample, comprise applying a first pretreating composition to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments, the first pretreatingcomposition comprises an aluminum oxide in water. For example, thealuminum oxide may comprise an additive, such as a nanoparticledispersion of aluminum oxide (for example, NanoBYK-3600®, sold by BYKAdditives & Instruments). The first pretreating composition may furthercomprise a surfactant or a wetting agent. The composite structure may beporous, allowing the pretreating composition to penetrate at least aportion of the pores of the composite structure.

In various embodiments, after applying the first pretreatingcomposition, the component may be heated to remove water and fix thealuminum oxide in place. For example, the component may be heatedbetween about 100° C. (212° F.) and 200° C. (392° F.), and further,between 100° C. (212° F.) and 150° C. (302° F.).

Step 215 may further comprise applying a second pretreating composition.In various embodiments, the second pretreating composition comprises aphosphoric acid and an aluminum phosphate, aluminum hydroxide, and/oraluminum oxide. The second pretreating composition may further comprise,for example, a second metal salt such as a magnesium salt. In variousembodiments, the aluminum to phosphorus molar ratio of the aluminumphosphate is 1 to 3 or less. Further, the second pretreating compositionmay also comprise a surfactant or a wetting agent. In variousembodiments, the second pretreating composition is applied to thecomposite structure atop the first pretreating composition. Thecomposite structure may then, for example, be heated. In variousembodiments, the composite structure may be heated between about 600° C.(1112° F.) and about 800° C. (1472° F.), and further, between about 650°C. (1202° F.) and 750° C. (1382° F.).

In various embodiments, step 215 may include applying a singlepretreating composition to an outer surface of a composite structurerather than a first pretreating composition and a second pretreatingcomposition, as discussed herein. The single pretreating composition maycomprise acid aluminum phosphate, which may in various embodiments, havea molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3. Invarious embodiments, the single pretreating composition may compriseacid aluminum phosphate with a molar ratio of aluminum to phosphatebetween 1 to 2 and 1 to 2.7. In various embodiments, the singlepretreating composition may comprise acid aluminum phosphate with amolar ratio of aluminum to phosphate of 1 to 2.5. In variousembodiments, the single pretreating composition may be substantiallyfree of aluminum oxide. As used herein “substantially free” meanscomprising less than 0.01% by weight of a substance. In variousembodiments, following the application of the single pretreatingcomposition, the composite structure may be heated to a temperaturesufficient to form an aluminum phosphate polymer layer on the compositestructure comprising aluminum orthophosphate (AlPO₄) and/or aluminummetaphosphate (Al(PO₃)₃). Such a temperature may be referred to as apretreating composition temperature. In various embodiments, thecomposite structure with the single pretreating composition may beheated between about 290° C. (554° F.) and about 900° C. (1652° F.), orbetween about 490° C. (914° F.) and about 625° C. (1157° F.), or betweenabout 600° C. (1112° F.) and about 800° C. (1472° F.), or between about650° C. (1202° F.) and about 750° C. (1382° F.). The single pretreatingcomposition may further comprise a surfactant or a wetting agent. Thecomposite structure may be porous, allowing the single pretreatingcomposition to penetrate at least a portion of the pores of thecomposite structure.

A single step of applying and/or heating the single pretreatingcomposition should cost less and take less time than applying a firstpretreating composition and, optionally, second pretreating composition,which would involve aluminum oxide, two application steps, and twoheating steps.

Step 215 may further comprise applying a barrier coating to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments, the barrier coatingcomposition may comprise carbides or nitrides, including at least one ofa boron nitride, silicon carbide, titanium carbide, boron carbide,silicon oxycarbide, and silicon nitride. In various embodiments, thebarrier coating may be formed by treating the composite structure withmolten silicon. The molten silicon is reactive and may form a siliconcarbide barrier on the composite structure. Step 215 may comprise, forexample, application of the barrier coating by spraying, chemical vapordeposition (CVD), molten application, or brushing the barrier coatingcomposition on to the outer surface of the carbon-carbon compositestructure. Any suitable manner of applying the base layer to compositestructure is within the scope of the present disclosure.

In various embodiments and with reference now to FIG. 2C, method 400 mayfurther comprise a step 240, similar to step 210, of forming a secondslurry by combining a second pre-slurry composition, which may comprisea second phosphate glass composition in glass frit or powder form, witha second carrier fluid (such as, for example, water). In variousembodiments, the second slurry may comprise an acid aluminum phosphatewherein the molar ratio of aluminum (Al) to phosphoric acid (H₃PO₄) maybe between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3,between 1:2.7 to 1:3 or between 1:2.9 to 1:3. In various embodiments,the second slurry may comprise a second pre-slurry compositionsubstantially free of phosphate glass, comprising acid aluminumphosphate and orthophosphoric acid with an aluminum to phosphate molarratio of 1:2 to 1:5. As used herein “substantially free” meanscomprising less than 0.01% by weight of a substance. Further, step 240may comprise spraying or brushing the second slurry of the secondphosphate glass composition on to an outer surface of the base layer. Invarious embodiments, the second slurry may be applied directly to thelayer formed by the pretreating composition(s) and/or barrier layer ofstep 215, such as an aluminum phosphate polymer layer formed from thesingle pretreating composition discussed herein, such that the oxidationprotection system does not include a base layer formed by heating afirst slurry. Any suitable manner of applying the sealing layer iswithin the scope of the present disclosure.

In various embodiments, the second slurry may be substantially free ofboron nitride. In this case, “substantially free” means less than 0.01percent by weight. For example, the second pre-slurry composition maycomprise any of the components of the pre-slurry compositions and/orglass compositions described in connection with the first pre-slurrycomposition and/or first phosphate glass composition, without theaddition of a boron nitride additive. In various embodiments, the secondpre-slurry mixture may comprise the same pre-slurry composition and/orphosphate glass composition used to prepare the first pre-slurrycomposition and/or the first phosphate glass composition. In variousembodiments, the second pre-slurry composition may comprise a differentpre-slurry composition and/or phosphate glass composition than the firstpre-slurry composition and/or first phosphate glass composition.

In various embodiments, the first slurry and/or the second slurry maycomprise an additional metal salt. The cation of the additional metalsalt may be multivalent. The metal may be an alkaline earth metal or atransition metal. In various embodiments, the metal may be an alkalimetal. The multivalent cation may be derived from a non-metallic elementand/or a metalloid element such as boron. The metal of the additionalmetal salt may be an alkaline earth metal such as calcium, magnesium,strontium, barium, or a mixture of two or more thereof. The metal forthe additional metal salt may be iron, manganese, tin, zinc, or amixture of two or more thereof. The anion for the additional metal saltmay be an inorganic anion such as a phosphate, halide, sulfate ornitrate, or an organic anion such as acetate. In various embodiments,the additional metal salt may be an alkaline earth metal salt such as analkaline earth metal phosphate. In various embodiments, the additionalmetal salt may be a magnesium salt such as magnesium phosphate. Invarious embodiments, the additional metal salt may be an alkaline earthmetal nitrate, an alkaline earth metal halide, an alkaline earth metalsulfate, an alkaline earth metal acetate, or a mixture of two or morethereof. In various embodiments, the additional metal salt may bemagnesium nitrate, magnesium halide, magnesium sulfate, or a mixture oftwo or more thereof. In various embodiments, the additional metal saltmay comprise: (i) magnesium phosphate; and (ii) a magnesium nitrate,magnesium halide, magnesium sulfate, or a mixture of two or morethereof.

The additional metal salt may be selected with reference to itscompatibility with other ingredients in the first slurry and/or thesecond slurry. Compatibility may include metal phosphates that do notprecipitate, flocculate, agglomerate, react to form undesirable species,or otherwise become segregated from the first slurry prior toapplication of the first slurry and/or the second slurry to thecarbon-carbon composite. The phosphates may be monobasic (H₂PO₄ ⁻),dibasic (HPO₄ ⁻²), or tribasic (PO₄ ⁻³). The phosphates may be hydrated.Examples of alkaline earth metal phosphates that may be used includecalcium hydrogen phosphate (calcium phosphate, dibasic), calciumphosphate tribasic octahydrate, magnesium hydrogen phosphate (magnesiumphosphate, dibasic), magnesium phosphate tribasic octahydrate, strontiumhydrogen phosphate (strontium phosphate, dibasic), strontium phosphatetribasic octahydrate and barium phosphate.

In various embodiments, a chemical equivalent of the additional metalsalt may be used as the additional metal salt. Chemical equivalentsinclude compounds that yield an equivalent (in this instance, anequivalent of the additional metal salt) in response to an outsidestimulus such as, temperature, hydration, or dehydration. For example,equivalents of alkaline earth metal phosphates may include alkalineearth metal pyrophosphates, hypophosphates, hypophosphites andorthophosphites. Equivalent compounds include magnesium and bariumpyrophosphate, magnesium and barium orthophosphate, magnesium and bariumhypophosphate, magnesium and barium hypophosphite, and magnesium andbarium orthophosphite.

While not wishing to be bound by theory, it is believed that theaddition of multivalent cations, such as alkaline earth metals,transition metals, nonmetallic elements, and/or metalloids such asboron, to the first slurry and/or the second slurry enhances thehydrolytic stability of the metal-phosphate network. In general, thehydrolytic stability of the metal-phosphate network increases as themetal content increases, however a change from one metallic element toanother may influence oxidation inhibition to a greater extent than avariation in the metal-phosphate ratio. The solubility of the phosphatecompounds may be influenced by the nature of the cation associated withthe phosphate anion. For example, phosphates incorporating monovalentcations such as sodium orthophosphate or phosphoric acid (hydrogencations) are very soluble in water while (tri)barium orthophosphate isinsoluble. Phosphoric acids can be condensed to form networks but suchcompounds tend to remain hydrolytically unstable. Generally, it isbelieved that the multivalent cations link phosphate anions creating aphosphate network with reduced solubility. Another factor that mayinfluence hydrolytic stability is the presence of —P—O—H groups in thecondensed phosphate product formed from the first slurry and/or thesecond slurry during thermal treatment. The first slurry and/or thesecond slurry may be formulated to minimize concentration of thesespecies and any subsequent hydrolytic instability. Whereas increasingthe metal content may enhance the hydrolytic stability of the firstslurry and/or the second slurry, it may be desirable to balancecomposition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in thefirst slurry and/or the second slurry at a concentration in the rangefrom about 0.5 weight percent to about 30 weight percent, and in variousembodiments from about 0.5 weight percent to about 25 weight percent,and in various embodiments from about 5 weight percent to about 20weight percent. In various embodiments, a combination of two or moreadditional metal salts may be present at a concentration in the rangefrom about 10 weight percent to about 30 weight percent, and in variousembodiments from about 12 weight percent to about 20 weight percent.

Method 400 may further comprise a step 250 of heating the compositestructure to a temperature sufficient to form a sealing layer, which maycomprise phosphate glass. In various embodiments, the sealing layer maybe formed over the layer formed by the pretreating composition and/orbarrier layer of step 215, such as an aluminum phosphate polymer layerformed from the single pretreating composition discussed herein, suchthat the oxidation protection system does not include a base layerformed by heating a first slurry. In various embodiments, the sealinglayer may be formed over the base layer. Similar to step 230, thecomposite structure may be heated to a temperature sufficient to adherethe sealing layer to the base layer or the layer formed by thepretreating composition and/or barrier layer (e.g., the aluminumphosphate polymer layer). Such a temperature may be referred to as asealing layer temperature. For example, step 250 may comprise drying orbaking the carbon-carbon composite structure at a temperature in therange from about 200° C. (392° F.) to about 1000° C. (1832° F.). Invarious embodiments, the composite structure is heated to a temperaturein a range from about 600° C. (1112° F.) to about 1000° C. (1832° F.),or between about 200° C. (392° F.) to about 900° C. (1652° F.), orfurther, between about 400° C. (752° F.) to about 850° C. (1562° F.),wherein in this context only, the term “about” means plus or minus 10°C. Further, step 250 may, for example, comprise heating the compositestructure for a period between about 0.5 hour and about 8 hours, wherethe term “about” in this context only means plus or minus 0.25 hours.

In various embodiments, step 250 may comprise heating the compositestructure to a first, lower temperature (for example, about 30° C. (86°F.) to about 300° C. (572° F.)) followed by heating at a second, highertemperature (for example, about 300° C. (572° F.) to about 1000° C.(1832° F.)). Further, step 250 may be performed in an inert environment,such as under a blanket of inert or less reactive gas (e.g., nitrogen,argon, other noble gases, and the like).

TABLE 1 illustrates a variety of phosphate glass compositions inaccordance with various embodiments.

TABLE 1 Phosphate Glass Composition A B C D E F Wt % Glass 75.01 75.0176.71 69.74 80.20 73.22 Wt % Boron Nitride 0 0 21.07 28.09 17.53 24.54Wt % o-AlPO4 0 2.27 0 0 0 0

Phosphate glass compositions A and B comprise boron nitride-freephosphate glasses. For example, glasses A and B may be suitable sealinglayers, such as the sealing layer applied in step 240 of method 400.Phosphate glass compositions C through F comprise boronnitride-containing phosphate glass. For example, glass compositions Cthrough F may illustrate suitable base layers, such as base layersformed by heating a first slurry in step 230 of methods 200, 300, and400. As illustrated, the boron nitride content of glass compositions Cthrough F varies between about 17.53 and 28.09 weight percent boronnitride. However, any suitable boron nitride-containing phosphate glass(as described above) is in accordance with the present disclosure.

With reference to FIG. 3 and TABLE 2 (below), experimental data obtainedfrom testing various glass compositions in accordance with variousembodiments is illustrated.

TABLE 2 Base Layer (none) E D C F Exposure Sealing Layer Oxidation TimeA A A A A Temp (Hours) Percentage Weight Loss 675 Degrees 0 0.00 0.000.00 0.00 0.00 C. 4 0.90 0.32 0.41 0.44 0.18 8 1.98 0.70 1.01 1.07 0.3912 3.13 1.19 1.76 1.95 0.64 16 4.41 1.80 2.56 2.98 0.95 20 5.94 2.523.56 4.65 1.31 24 7.48 3.41 4.73 6.29 1.76 760 Degrees 26 10.63 4.316.43 8.47 2.11 C. 28 14.28 5.40 8.37 11.13 2.54 870 Degrees 30 24.437.18 11.96 15.95 3.50 C. 32 36.45 12.76 17.27 23.43 6.89

The base layer and sealing layers shown in TABLE 2 are the same as shownin TABLE 1, with like labeling A through F. As illustrated in TABLE 2, abase layer of phosphate glass formed of boron nitride-comprising glasscompositions C through F are applied as a slurry (i.e., a first slurryof a first phosphate glass composition, such as that formed in step 210in FIGS. 2A-2C) to a composite structure. A sealing layer formed ofglass composition A is applied over the base layer (i.e., a secondslurry of a second phosphate glass composition, such as that formed instep 240 in FIG. 2C). As shown, the composite structure having a baselayer exhibited a lower weight loss to oxidation at temperatures at andabove 675° C. (1250° F.) than composite structures having sealing layerA by itself. FIG. 3 illustrates the data from TABLE 2, with percentweight loss on the y axis and exposure time on the x axis.

With reference to FIG. 4, a number of combinations are illustrated,including various combinations of pre-treatment (such as, for example,as may be performed by pretreatment step 215 with a first pretreatingcomposition and/or second pretreating composition), base layers, andsealing layers. The base layer and sealing layers are shown in FIG. 4with the same labeling as shown in TABLE 1, A through F. For example,FIG. 4 illustrates that the performance of a composition comprising abase layer of glass composition F and a sealing layer B providesimproved oxidation protection over a pretreated composite structurehaving only sealing layer B. Other combinations include a base layer ofglass composition F and a sealing layer A (without pre-treatment), apretreatment with sealing layer A only, a base layer of F with a sealinglayer of A (with pre-treatment).

TABLE 3 illustrates a variety of slurries comprising phosphate glasscompositions and prepared in accordance with various embodiments.

TABLE 3 Example A B1 B2 C D E F G H h-Boron nitride powder A 0 0 0 8.758.75 8.25 8.25 8.25 8.25 h-Boron nitride powder B 0 0 0 0 0 0.5 0.5 0.50.5 Graphene nanoplatelets 0 0 0 0.15 0.15 0.15 0.15 0.15 0.15 H₂O 52.452.4 52.4 50.0 50.0 45.0 45.0 60.0 60.0 Surfynol 465 surfactant 0 0 00.2 0.2 0.2 0.2 0.2 0.2 Ammonium dihydrogen phosphate 11.33 11.33 1.0 00 0 0 0.5 0 (ADHP) NH₄OH 0 0 0 0.5 0 0.5 0.5 0 0.5 Glass frit 34.0 34.034.0 26.5 26.5 26.5 26.5 26.5 26.5 Acid aluminum phosphate (1:3 Al—P) 00 0 0 0 10.0 0 0 0 Acid aluminum phosphate (1:2.1 Al—P) 0 0 10.0 0 0 0 05.0 0 Acid aluminum phosphate (1:2.5 Al—P) 0 0 0 0 0 0 0 0 5.0 Acidaluminum phosphate (1:2 Al—P) 0 0 0 0 0 0 10.0 0 0 Ammonium dihydrogenphosphate 0 0 0 0 0.5 0 0 0 0 (ADHP) Aluminum orthophosphate (o-AlPO₄) 02.27 2.27 0 0 0 0 0 0

TABLE 4 illustrates a variety of acid aluminum phosphate solutions inaccordance with various embodiments.

TABLE 4 Acid aluminum phosphate solutions Component MW AlPO 1:4 AlPO1:3.5 AlPO 1:3 AlPO 1:2.5 AlPO 1:2 Amount (grams) H₂O 18.01 60.10 60.6561.42 62.50 64.08 H₃PO₄ (85%) 98.00 100.00 100.00 100.00 100.00 100.00Al(OH)₃ 78.00 19.90 22.74 26.53 31.85 39.80 Al:P (molar ratio) 0.2500.286 0.333 0.400 0.500

As illustrated in TABLES 3 and 4, oxidation protection system slurriescomprising a phosphate glass composition glass frit in a carrier fluid(i.e., water) and various additives including h-boron nitride, graphenenanoplatelets, a surfactant, a flow modifier such as, for example,polyvinyl alcohol, polyacrylate or similar polymer, ammonium dihydrogenphosphate, ammonium hydroxide, and acid aluminum phosphates withAl:H₃PO₄ molar ratios of between 1 to 2 and 1 to 3 were prepared. Suchas, for example, slurry example H contained h-boron nitride and an acidaluminum phosphate solution with an aluminum to phosphorus molar ratioof 1:2.5. Phosphate glass compositions in slurries A, B1, and B2comprise boron nitride-free phosphate glasses. For example, glasses A,B1, and B2 may be suitable sealing layers, such as the sealing layerformed by the application of second slurries in step 240 of method 400.Phosphate glass compositions C through H comprise boronnitride-containing phosphate glass. For example, glass compositions Cthrough H may illustrate suitable base layers, such as base layersformed by the application of first slurries in step 210 of methods 200,300, and 400.

With combined reference to TABLE 3 and FIG. 5, the first slurries(examples C, D, E, F, and H) were applied to 50 gram carbon-carboncomposite structure coupons and cured in inert atmosphere under heat at899° C. (1650° F.) to form base layers. After cooling, second slurries(examples A, B1 or B2) were applied atop the cured base layer and thecoupons were fired again in an inert atmosphere. A control coupon waspretreated with an alumina nanoparticle (i.e., a first pretreatingcomposition as discussed in association with step 215) and given an acidaluminum phosphate layer (i.e., a second pretreating composition asdiscussed in association with step 215) with a Al:H₃PO₄ molar ratio ofabout 1 to 3.3, as described in various embodiments, and cured under aninert atmosphere. A control was prepared with a second slurry comprisinga phosphate glass composition, water, ammonium dihydrogen phosphate, andaluminum orthophosphate. The second slurry for the control (example B1)was applied atop the cured pretreated control and then fired again underan inert atmosphere forming, a sealing layer. After cooling, the couponswere subjected to isothermal oxidation testing a 760° C. (1400° F.) overa period of hours while monitoring mass loss.

With further reference to TABLE 3 and FIG. 5, the performance of thecoatings applied according to various embodiments is illustrated incomparison with a control. The control includes a pretreated compositestructure having only sealing layer from slurry B1. Percent weight lossis shown in the y axis and exposure time is shown in the x axis. Againstthe control, the addition of an acid aluminum phosphate with molar ratioof Al:H₃PO₄ of between 1:2 and 1:3 reduces mass losses due to oxidationby between two times to over ten times (i.e., an order of magnitude).After 24 hours at 760° C. (1400° F.) the control had lost 24.1% of itsmass in comparison the best performing test sample which had lost only0.7% of its mass. The combined effect of adding acid aluminum phosphateinto the slurry wherein the molar ratio of Al:H₃PO₄ is between about 1:2and 1:3 provides an unexpected increase in protection over standalonepretreatment of the carbon-carbon composite structure followed byapplication of a base layer or a base layer and sealing layer. Withoutbeing bound by theory, it is thought that the addition of acid aluminumphosphate wherein the molar ratio of Al:H₃PO₄ is between about 1:2 and1:3 produces a base layer which is both cohesive and adhesive.

With reference to Table 5 (below) and FIG. 6, experimental data obtainedfrom testing various oxidation protection systems in accordance withvarious embodiments is illustrated.

TABLE 5 Data Set 610 605 Pretreatment Single pretreating First andSecond pretreating composition compositions Base Layer (none) (none)Exposure Sealing Layer Oxidation Time B1 B1 Temp (Hours) PercentageWeight Loss 760 Degrees 0 0.00 0.00 C. 4 0.39 0.38 8 1.68 1.56 12 4.414.63 16 8.47 9.57 20 13.08 15.94 24 20.90 24.11

Table 5 and FIG. 6 show the performance of pretreatment compositionsdiscussed herein in relation to step 215 in FIGS. 2B and 2C, which maybe applied to a composite structure prior to the application of a firstslurry (step 210) and/or a second slurry (step 240). Percent weight lossof the carbon structure due to oxidation is shown on the y-axis, andexposure time to a temperature of at 760° C. (1400° F.) in hours isshown on the x-axis.

Data sets 605 and 610 in Table 5 and FIG. 6 show experimental data forcomposite structures with oxidation protection systems comprisingpretreatment compositions and a sealing layer formed by a second slurryB1 (in Table 3). Data set 605 displays experimental data for a compositestructure that had been pretreated with a first pretreating compositioncomprising aluminum oxide and a second pretreating compositioncomprising acid aluminum phosphate with an aluminum to phosphate molarratio of 1 to 3.3, as discussed in relation to step 215 in FIGS. 2B and2C, before slurry B1 was applied to the composite structure. Data set610 displays experimental data for a composite structure that had beenpretreated with a single pretreating composition comprising acidaluminum phosphate with an aluminum to phosphate molar ratio of 1 to2.5, as discussed in relation to step 215 in FIGS. 2B and 2C, beforeslurry B1 was applied to the composite structure. After 24 hours at 760°C. (1400° F.) the composite structure reflected by data set 605 had lost24.11% of its mass, while the composite structure reflected by data set610 had lost 20.90%. Therefore, the effect of a single pretreatingcomposition, as described herein, being applied to a composite structureprovides an unexpected increase in oxidation protection for thecomposite structure from a pretreatment comprising a first pretreatingcomposition and a second pretreating composition. Additionally, asdiscussed herein, pretreatment of a composite structure involving asingle pretreating composition allows for cost and time savings asopposed to the pretreatment involving the first pretreating compositionand/or second pretreating composition.

Table 6 (below) shows experimental data obtained from testing variousoxidation protection systems comprising a substantially phosphateglass-free sealing slurry comprising acid aluminum phosphate andorthophosphoric acid, in accordance with various embodiments.

TABLE 6 Base Layer G G Exposure Sealing Layer Oxidation Time (none) JTemp (Hours) Percentage Weight Loss 760 Degrees 0 0.00 0.00 C. 4 8.340.50 8 23.10 2.15 12 43.35 5.55 16 — 11.03 20 — 17.82 24 — 25.84

As illustrated in TABLE 6, oxidation protection systems with variousslurries were tested for effectiveness at preventing oxidation oncomposite structures. Slurry example G (from Table 3) contained a firstpre-slurry composition comprising a first phosphate glass compositionand various additives including h-boron nitride, graphene nanoplatelets,a surfactant, ammonium hydroxide, and acid aluminum phosphate withAl:H₃PO₄ a molar ratio of 1 to 2.1. The first pre-slurry compositionfrom slurry example G was combined with a carrier fluid (i.e., water).Slurry G was applied to 50 gram carbon-carbon composite structurecoupons and cured in inert atmosphere under heat at 899° C. (1650° F.).After cooling, a second slurry J comprising acid aluminum phosphateand/or orthophosphoric acid with a molar ratio of aluminum to phosphateof between 1 to 2 and 1 to 5 was applied atop the cured base layer andthe coupons were fired again in an inert atmosphere. Second slurry J issubstantially free of phosphate glass.

With further reference to TABLE 6, the performance of the sealing slurryJ, which creates a sealing layer in response to being heated in step250, applied to a composite structure according to various embodimentsis illustrated in comparison with a control. The control includes apretreated composite structure having only base layer G. Against thecontrol, the addition of the second slurry J reduces mass losses due tooxidation by between five times to over ten times (i.e., an order ofmagnitude). After 12 hours at 760° C. (1400° F.) the control had lost43.35% of its mass in comparison to the composite structure with baselayer G and sealing layer J, which lost only 5.55% of its mass. Theeffect of adding a substantially phosphate glass-free second slurry(i.e., slurry J) to the composite structure comprising acid aluminumphosphate and orthophosphoric acid wherein the molar ratio of Al:PO₄ isbetween about 1:2 and 1:5 provides an unexpected increase in protectionover standalone pretreatment of the carbon-carbon composite structurefollowed by application of only a base layer.

Benefits and other advantages have been described herein with regard tospecific embodiments. Furthermore, the connecting lines shown in thevarious figures contained herein are intended to represent exemplaryfunctional relationships and/or physical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present in apractical system. However, the benefits, advantages, solutions toproblems, and any elements that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed ascritical, required, or essential features or elements of the disclosure.The scope of the disclosure is accordingly to be limited by nothingother than the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” Moreover, where a phrase similar to“at least one of A, B, or C” is used in the claims, it is intended thatthe phrase be interpreted to mean that A alone may be present in anembodiment, B alone may be present in an embodiment, C alone may bepresent in an embodiment, or that any combination of the elements A, Band C may be present in a single embodiment; for example, A and B, A andC, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detaileddescription herein, references to “one embodiment,” “an embodiment,” “anexample embodiment,” etc., indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed. After reading the description, it will be apparent to oneskilled in the relevant art(s) how to implement the disclosure inalternative embodiments.

Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element is intended to invoke 35 U.S.C. 112(f)unless the element is expressly recited using the phrase “means for.” Asused herein, the terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus.

1. An article comprising: a carbon-carbon composite structure; and anoxidation protection system including: an aluminum phosphate polymerlayer disposed on an outer surface of the carbon-carbon compositestructure, the aluminum phosphate polymer layer having a first molarratio of aluminum to phosphate between 1 to 2 and 1 to 3; a base layerdisposed on the aluminum phosphate polymer layer, wherein the base layercomprises a first pre-slurry composition comprising a first phosphateglass composition; and a sealing layer disposed on the base layer,wherein the sealing layer comprises a sealing pre-slurry compositioncomprising a second phosphate glass composition.
 2. The article of claim1, wherein the base layer comprises a second molar ratio of aluminum tophosphate between 1 to 2 and 1 to 3.